Snails are plentiful in some regions. They can also be introduced and raised on agricultural waste provided that they cannot escape and become a threat for the environment. They can be collected and processed into a viable supplementary source of protein that can be used to replace other animal protein sources in feed rations.

A notable snail species that can be used to feed livestock is the golden apple snail (Pomacea canaliculata and other Ampullariidae species).

Distribution

The golden apple snail (Pomacea canaliculata) is a freshwater snail indigenous to South America. It was introduced into South-East Asia as an aquarium species in 1980. In Taiwan, it was introduced as a new human food resource (Mochida, 1991). It was later farmed as a high-protein food for both humans and farm animals, particularly for ducks and fish.

It escaped into waterways and soon became a major pest of rice in many countries, notably into the Philippines and Vietnam, damaging young rice seedlings and causing poor crop stands, yield losses and additional expenses. Six years after its introduction in the Philippines, the golden apple snail had invaded about 3.6% of the total area planted with rice (GISD, 2012; Cagauan et al., 2002a). It has also invaded the southern parts of the USA as well as Hawaii, where it is a major pest of taro cultivation. In addition to being a crop pest, it poses a serious threat to many wetlands through potential habitat modification and competition with native species. Total eradication of established populations is nearly impossible. In rice and taro fields, one of the most successful methods is hand picking done as a community effort on a regular basis. After collection the snails can be used for human consumption or as animal feed for fish, prawns, pigs and other livestock species (GISD, 2012).

Processes

There are different ways to prepare snails for animal feeding.

Apple snails can be fed live, freshly dead or processed. Live snails are eaten directly by fish and ducks in ponds and rice fields. Collected snails can be fed whole or without shells (snail meat). Whole snails or snail meat is fed fresh or ensiled, cooked and/or dried. Snail shells can be a source of minerals similar to oyster shells. A kg of whole snails, when washed and unshelled, yields about 250 g of fresh snail meat and 100 g of dried snail meat (Ulep et al., 1991). Because fresh snail meat spoils easily, cooking, drying or ensiling should be considered whenever the snails cannot be eaten immediately.

Fresh snail meat can be prepared as follows: the snails are cleaned, washed and crushed and the shells are separated from the meat (Salazar et al., 2003). A preliminary step may involve purging the snails for two days (Ulep et al., 1995).

Snail meal

Various methods for producing snail meal have been described in the literature. Some authors recommend boiling the snails first in order to kill them, to remove pathogens and to facilitate the separation of the meat from the shell (Ulep et al., 1991). The resulting snail meat can be fed fresh or be sun-dried/oven-dried and then ground.

It is important to note that while apple snails are numerous, purchasing snail meat can be expensive because the price includes the labour costs of removing the shells and chopping. In Laos, this cost was estimated at 27% of the total cost of pig diets. Farmers who collect and process the snails themselves may get a more direct economic benefit than those who buy snail meat (Kaensombath et al., 2005a).

In the papers cited in the rest of this contribution, apple snail meal refers to snail meat and does not include the shells unless stated otherwise.

Fresh snail meal

Snails are cleaned, washed and then crushed. Shells are separated from the meat. The meat is ready to be mixed with fish meal or soybean oil meal (Salazar et al., 2003).

Dried snail meal

After removing from the shell, the visceral mass is washed in alum solution to remove slime, then washed again, cut into small pieces and transferred into an oven to dry at 60-70°C for 72h. It can then be ground.

Ensiled snail meal

Making silage from apple snail meat has been described as a low cost and simple method for small-scale production (Kaensombath et al., 2005c). Many ensiling processes have been assessed and are described hereafter:

After snail meat extraction, washing in clear water, drainage and chopping into 0.5-1.0 cm long pieces, snail meal was complemented with a source of carbohydrates (1 kg per 1, 2 or 3 kg snail meat) such as molasses and/or rice bran to start the ensiling process. Silage was then stored for 24 weeks (Kaensombath et al., 2005a).

After meat extraction, chopping and grinding, the addition of 15-20% molasses resulted, after 28 days in closely tight plastic bags, in satisfactory silage with a brownish colour and a pleasant smell (Phonekhampheng et al., 2009). Snails ensiled with citric acid deteriorated rapidly.

After boiling whole snails in water for 2 min, meat was extracted, minced and blanched for 5 min, and then mixed with molasses and inoculated with lactic acid bacteria. Silage was done during 15 days and pH dropped from 8 to 4 (Rattanaporn et al., 2006).

Environmental impact

Invasive species

The use in animal feeding of invasive snails species such as the golden apple snail is a way to control their development and limit their environmental impact.

Ducks are commonly used for the biological control of apple snails in paddies and taro patches. Both Mallards (Anas platyrhynchos) and Muscovy ducks (Cairina moschata) are used although the former is preferred by farmers (Serra, 1997). Ducks can be introduced to the fields after harvest and be removed from it before rice transplanting, or they can be introduced to the paddies three weeks after transplanting. Duck stocking rates suggested for snail control are highly variable, ranging from 5-10 ducks/ha in Malaysia (Teo, 2001) to 200, 400 and even 900 ducks/ha in the Philippines (Pantua et al., 1992; Vega, 1991; Cagauan, 1999; Cagauan et al., 2002a). In China, young ducks readily eat young snails (hatchlings and juveniles) weighing less than 1.5 g, while 60 day old ducks are the main predators of older and adult snails (1.5 g to over 6.5 g) (Kaiming Liang et al., 2013).

Soil improver

Apple snail shells can improve acidic soils by adding lime to them.

Nutritional aspects

Nutritional attributes

Whole snails are relatively poor in protein, about 14-18% on a DM basis, particularly rich in calcium (28-31% DM), and poor in phosphorus (< 0.5% DM). At the time of writing, no data were available regarding apple snail minerals other than calcium and phosphorus.

Snail meat (without shells) contains 52-63% protein (DM). It is similar to a fish meal of moderate quality. Ash content varies between 11 and 27% DM, and depends on the amount of residual shell material included. Snail meat contains about 3-4% calcium and 0.4-1.2% phosphorus. Fat content is generally less than 5%, much less than that of a typical fish meal (about 7-14%). Shells are mostly mineral matter and contain about 35% calcium with minimum amounts of residual protein.

There are few published amino acid profiles of apple snail proteins. Unfortunately, these profiles are not very consistent with each other. The reported lysine content of the protein, for instance, ranges from 2.9 to 9.7% of crude protein, a range that is abnormally large for animal products. This could be explained by difficulties to analyse this unconventional material, or by differences in the tissue composition of the snail meat, which may include different proportions of muscle and viscera. The protein content could also be influenced by the presence of undigested matter in the gut. Until more data are available, it is therefore difficult to draw conclusions about the suitability of apple snail protein for animal feeding.

Potential constraints

Concentration of contaminants

Apple snails may concentrate dangerous pollutants from freshwater bodies, such as mercury, arsenic and uranium, in their midgut, kidney and foot. They are thus considered good bio-indicators for water contamination but unrestricted feeding by humans and animals might be considered with caution (Vega et al., 2012).

Snail meal

Fresh golden snail meal (Pomacea canaliculata) is a good source of protein for pigs in the Philippines, where whole and uncooked apple snail meal (with shells) introduced at 15% in the diet of growing pigs resulted in performance (average daily gain, feed conversion ratio) similar to that obtained with a commercial mash (Catalma et al., 1991a). It could replace 10% of a commercial mash in growing-finishing diets (Salazar et al., 2003). However, in another trial, whole snail meal at 50% had no effect on performance (Garcia, 2010).

Fresh or ensiled golden apple snail meat has been used to replace fish meal in growing (30-70 kg) pig diets. In a trial in Laos, the apparent digestibilities of crude protein and dry matter were about 81-83% and 55-59% respectively (Kaensombath et al., 2005a). In a feeding trial, total replacement of fish meal with fresh snail meat (9% and 5.5% of diet DM for 30-50 kg and 50-70 kg pigs, respectively) or ensiled snail meat (15.5% and 9.5% for 30-50 kg and 50-70 kg pigs, respectively) reduced diet DM intake, perhaps because of the high moisture content of the snails, but did not alter daily weight gain and feed conversion ratio. It was concluded that replacing fish meal with fresh or ensiled snail meat could be economically effective for pig production and could increase rice yields in the fields (Kaensombath et al., 2005b).

Fresh apple snail

Snail meal is a suitable substitute for more traditional protein sources in poultry diets. It can usually be added at 10-15% (diet DM). In chicks, feeding 10% of uncooked snail meal resulted in a 31% increase in total weight gain and 35% improvement in feed efficiency, compared to the control diet (Catalma et al., 1991b).

Broilers

In the Philippines, for broilers fed 12% cooked or raw snail meal, cooking improved the feed conversion ratio and the palatability of snail meal.

Boiled snail meal led to similar production results and higher intake in chicks when compared to fish meal (Venugopalan et al., 1976). Cooked snail meal led to better performance than raw snail meal, and to slightly lower performance than the fish meal based control diet (Barcelo et al., 1991).

Body weight and live-weight gains were similar for broilers fed a maize-soybean diet and broilers fed snail meal (Ali et al., 1995). Whole (including shells) dried giant snail meal was included in broilers at up to 6% of the diet, but the best results were obtained at the 2% inclusion level (El-Deek et al., 2002). Snail meal replaced up to 30% of the fish meal in starter phase of broilers, and up to 100% of the fishmeal in the grower stage, with an increase of growth rate and no negative effects on the taste of broiler meat (Diomandé et al., 2008).

Laying hens

Replacing fish meal in layer diets had no negative effects on egg numbers or egg quality, in addition of being cheaper than fish meal (Diomandé et al., 2008). Similarly, up to 15% golden snail meal was fed to layers without depressing performance. Layers performed best when snail meal was fed at the 10% level (Serra, 1997).

In the Philippines, studies with laying hens have produced contradictory results. Crushed snails given to White Leghorn layers as a supplement (20 g/bird/day) to a commercial mash resulted in a 88% mean hen-day egg production rate compared to 84% without the supplement (Ancheta, 1990). Also in the Philippines, ground snail meal included at 11% or 25% in layer diets resulted in lower hen-day egg production than for the control diet rate (72% and 84% respectively). However, feed intake, feed conversion, shell thickness and albumen weight were not affected, and feeding snail meal to layers resulted in a higher value of eggs (Catalma et al., 1991b).

In the Philippines, laying Mallards ducks fed fresh and crushed snails mixed with rice bran and broken maize grains at a ratio of 1.1:1 exhibited a 60-70% egg production rate (Tacio, 1987), while feeding ad libitum fresh snails and small amounts of rough rice resulted in a 68% egg production rate (Aquino, 1990). The use of a 2:1 ratio of fresh snails and rice bran has also been reported (Serrano, 1988). Mallards can be fed economically on a 50:50 mixture of apple snails and rice bran, and although ducks fed the snail and bran diet had a lower final body weight and feed efficiency than ducks fed on commercial diets, economic returns were higher (PCARRD, 2006). The combination of snails and commercial duck layer feeds at a ratio of 1:1.3 resulted in optimum egg production rate and low production cost (Datuin et al., 1990).

Fish

Good results have been obtained in several fish species raised for human consumption by replacing fish meal with snail meal.

In African catfish (Clarias gariepinus), snail meal ensiled with molasses could replace 100% of the fish meal in the diet (20-27% of the total diet DM) without affecting growth performance and feed utilization (Phonekhampheng et al., 2009).

Grouper

In tiger grouper (Epinephelus fuscoguttatus), apple snail meal could be used at up to 20% (DM basis) without affecting performance. Higher inclusion rates resulted in reduced growth and survival (Usman et al., 2007). Feeding cultured grouper (Epinephelus tauvina) with 100% golden apple snails (fresh, cooked or a 50:50 mixture) resulted in lower survival than with a fish meal diet, though the highest growth among the snail-only diets was obtained with a 50:50 mixture of fresh and cooked snails (Firdus et al., 2005).

Seabass

In seabass (Lates calcarifer), the replacement of fish meal by up to 25% of apple snail meal was found acceptable. Higher replacement rates decreased digestibility and performance (Hanafi, 2003).